The other monks have answered your question rather well, it seems, but I thought perhaps I might have a suggestion that could be of use as well. Maybe you could simply ask your users where they're from? This kind of thing would be trivial to implement, and would probably come across far less "sneaky" than inferring from logs. And if you make it optional, I think people would be more likely to simply put nothing instead of filling it with a false value. Also, once you have people actively announcing where they're from, you could correlate this information with their IP address in the future. :)

HTH